Photosynthesis is a fundamental biological process where organisms convert light energy into chemical energy. While often associated with plants, a variety of bacteria also perform photosynthesis. Bacterial photosynthesis is diverse and exhibits key differences from the process found in plants.
Distinct Pathways of Bacterial Photosynthesis
Bacterial photosynthesis occurs through two pathways: oxygenic and anoxygenic. Oxygenic photosynthesis utilizes water as an electron donor, releasing oxygen as a byproduct. It contributes to Earth’s atmospheric oxygen.
Conversely, anoxygenic photosynthesis does not produce oxygen. It uses electron donors other than water, such as hydrogen sulfide, various sulfur compounds, ferrous iron, or organic molecules. The specific electron donor determines the byproducts, which can include elemental sulfur. This process is common in environments where oxygen is scarce or absent.
Key Photosynthetic Bacterial Groups
Cyanobacteria are the primary group of bacteria performing oxygenic photosynthesis. They are widespread in aquatic and terrestrial environments and produce oxygen. Cyanobacteria contain chlorophyll a and accessory pigments like phycobilins.
Other bacterial groups engage in anoxygenic photosynthesis. Purple bacteria, like Chromatium and Rhodobacter, use bacteriochlorophylls and thrive in anaerobic conditions, using compounds like hydrogen sulfide or organic acids. Green sulfur bacteria, including Chlorobium, are obligate anaerobes that primarily use hydrogen sulfide as an electron source, depositing elemental sulfur. Heliobacteria are Gram-positive phototrophs containing bacteriochlorophyll g.
The Unique Chemistry of Bacterial Photosynthesis
Bacterial photosynthesis chemistry diverges from plants, particularly in light-harvesting pigments. Most anoxygenic photosynthetic bacteria employ bacteriochlorophylls instead of chlorophylls. These pigments are structurally similar but absorb different light wavelengths, often in infrared or far-red regions. For example, bacteriochlorophylls absorb light in specific infrared or far-red ranges.
Another chemical distinction is the diverse electron donors used by anoxygenic bacteria. Unlike oxygenic photosynthesis, which uses water, anoxygenic bacteria draw electrons from various reduced compounds. These include inorganic substances like hydrogen sulfide, thiosulfate, or ferrous iron, as well as organic acids. The specific electron donor dictates the end products of the photosynthetic reaction, often resulting in the formation of sulfur or sulfate compounds instead of oxygen. Bacterial photosynthetic reaction centers also typically feature one photosystem compared to the two found in oxygenic organisms.
Ecological and Evolutionary Significance
Photosynthetic bacteria play significant roles in Earth’s ecosystems, shaping the planet’s history. They are primary producers, converting light energy and carbon dioxide into organic matter, especially in environments where plants cannot thrive, like anoxic sediments and hot springs. They contribute to global carbon fixation and nutrient cycling, particularly the sulfur cycle, by oxidizing sulfur compounds.
From an evolutionary perspective, anoxygenic bacterial photosynthesis evolved before oxygenic photosynthesis. Early Earth’s atmosphere lacked oxygen, making anoxygenic pathways well-suited for those conditions. The later evolution of oxygenic photosynthesis, particularly by cyanobacteria, led to the Great Oxidation Event, when atmospheric oxygen levels significantly increased. This change in atmospheric composition paved the way for the evolution of more complex, oxygen-dependent life forms, including plants and animals.